i used a 70-200mm with a 7D for the singapore zoo and it was a little too much as most of the animals are extremely close to you, well except for the tigers and orangutans enclosures, so to me it was a little too much for me. Eventually i went again with family members and this time brought a 15-85mm and it was just nice. If i needed head shots, i can just crop them. I guess eventually we have to decide if the subjects that you are shooting large or small, if its some small bird, then ring the 70-200mm, but if its like elephants and such, it was a little too tight for me as i had to constantly move myself back away from the enclosure just to shoot the animals. Hope it helps.